Multiple temperature threshold sensing having a single sense element

ABSTRACT

A temperature sensing system ( 30 ) that providing at least one detect signal related to temperature in an integrated circuit is presented. This system uses one thermal sensing circuit ( 40 ) to detect two or more temperature thresholds (t 2 , t 3 ) and differentiates the temperature thresholds using time multiplexed logic control. The system ( 80 ) capable of detecting more than two temperatures includes the thermal sensing circuit ( 90 ) and a decode circuit ( 92 ) with at least one detect latch ( 100 ). Optionally, the system may include a hysteresis circuit ( 60 ). The thermal sensing circuit ( 40 ), connected to the integrated circuit, generates a detect signal (D 4 ) in response to the a temperature selection signal (T 1 ). This flexible on-board temperature monitoring solution reduces the cost of thermal feedback sensing by reducing die area and improves the correlation of detected temperatures. In addition, this solution reduces the possibility of mismatch and tracking errors between two or more sense elements.

FIELD OF THE INVENTION

This invention relates generally to the field of temperature threshold sensing; and, in particular, to a multiple temperature threshold sensing circuit having a single sense element.

BACKGROUND OF THE INVENTION

Over the past two decades, the semiconductor industry has greatly advanced from incorporating a few transistors on one integrated circuit chip to incorporating millions of transistors. The integrated circuit is at the heart of most electronic equipment today, e.g. navigational systems, computers, pocket calculators, industrial monitoring and control systems, digital watches, digital sound systems, word processors, communications networks, and innumerable others. The vast number of transistors on a small area of semiconductor material has its advantages in speed, reliability, and negligible weight, but has its disadvantages in power consumption. More specifically, due to the increase in power consumed by each transistor, there exists a cumulative effect of temperature rise.

Conventional systems use two forms of cooling systems: passive and active. These cooling systems are mounted a circuit board that includes the integrated circuit package. Passive cooling involves the use of a heat sink. This form of cooling however has limited capacity to dissipate heat and increases the weight of the complete circuit board module. Active cooling involves the use of a device such as a fan which pulls air over the package to cool the die. Fans are not efficient because they require more space and power. In addition, fans are not desirable because they generate noise.

In addition to the use of cooling systems, thermal sensing systems are used to monitor the temperature of the integrated circuit. More specifically, they monitor portions of the integrated circuit having specific functions within an electronic system to determine when the temperature exceeds a predetermined temperature threshold. Once the integrated circuit has exceeded the predetermined temperature threshold, that particular portion of the integrated circuit having the specified function is shut down. One such thermal sensing system comprises a thermocouple attached to a heat sink. Another thermal sensing system includes a diode or a bipolar transistor and an external analog circuit. Since the current and voltage characteristics of a diode are temperature dependent, the external analog circuit is used to track the current and voltage characteristics of the diode, while simultaneously monitoring the temperature of the diode. Once the temperature has exceeded a particular value, the external analog circuit generates a signal to trigger the shutting down of that particular function of the electronic system. For a sensing system which includes a bipolar transistor in lieu of the diode, the external analog circuit monitors the base-emitter voltage V_(be) of the transistor since the reference voltage of the bipolar transistor is temperature dependent.

Some thermal sensing systems include hysteresis logic. One such implementation may include a current mirror coupled to a thermal sense element, such as a diode or bipolar transistor. A current reference, having a current proportional to the absolute temperature, generates a bias current which is applied to the current mirror. The bias current and resistance within the thermal sensing circuit is predefined such that the thermal sense element conducts current at a particular predetermined temperature threshold. A typical pre-determined shutdown threshold may be 150° C. In the instance where the thermal element is a bipolar transistor, as noted above, the reference voltage of the bipolar transistor is quite temperature dependent. Its base-emitter voltage V_(be) has a negative temperature coefficient of approximately 1.5 to 2.5 mV/° C. As temperature increases, the base-emitter voltage V_(be) necessary to turn on the bipolar transistor decreases. When the temperature of the circuit reaches the predetermined temperature threshold, the bipolar transistor begins to conduct current. The hysteresis logic switches in additional current applied to the base of the bipolar transistor after the predetermined temperature threshold is reached.

This thermal sense system for protecting an integrated circuit from overheating is successful; however, in many systems, it is desirable to keep all functions available to the end-user. Setting a thermal sense flag eliminates the need to shut down the function entirely. A second thermal sense circuit with a lower detect threshold creates this feature. However, utilizing separate sense elements introduces a semiconductor matching problem where the coefficients associated with the process, die stress, and thermal gradients have a high probability of not being equivalent. In addition, adding a second thermal sense circuit requires a greater portion of die area to incorporate redundant current mirroring required for multiple circuits. This problem escalates when sensing more than two distinct thresholds. Thus, there is a need for a thermal sensing system that utilizes one thermal sensing element having the capability to sense two or more distinct thresholds.

SUMMARY OF THE INVENTION

A temperature sensing system provides at least one detect signal related to temperature in an integrated circuit. This system uses one analog thermal sensing circuit to detect two or more temperature thresholds and differentiates the temperature thresholds using a time multiplexed logic control. The system includes the thermal sensing circuit and a decode circuit with at least two detect latches. Optionally, the system may include a hysteresis circuit. This thermal sensor circuit, connected to the integrated circuit, generates a detect signal in response to a temperature selection signal. This flexible on-board temperature monitoring solution reduces the cost of thermal feedback sensing by reducing die area and improves the correlation of detected temperatures. This solution reduces the possibility of mismatch and tracking errors between two or more sense elements.

There are numerous advantages of the present apparatus of sensing temperature at multiple thresholds over previous implementations. First, the use of a single sense element minimizes the possibility of mismatch and tracking errors between two or more sense elements. This implementation reduces the relative error to current mirror mismatch. The simple time multiplexed decode of the detect signal saves die area verses the implementation of redundant sense circuits; thus, it provides a cost effective solution. Most importantly, with the addition of relatively few components, a time multiplexed method of sensing die temperature results in a flexible on-board temperature monitor.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein:

FIG. 1 is a schematic of a known thermal sense circuit;

FIG. 2 is a block diagram of a known thermal sense circuit having multiple sense elements;

FIG. 3 is a block diagram of an embodiment of a time multiplexed multiple threshold sensing system with a single sense element in accordance with the present invention;

FIG. 4 is a schematic of an embodiment of a thermal sense circuit in accordance with the present invention;

FIG. 5 is a schematic of an embodiment of a detect logic circuit in accordance with the present invention;

FIG. 6 is a timing/temperature multiplexed diagram of dual temperature threshold sensing of the threshold sensing system of FIG. 3;

FIG. 7 is a schematic of an embodiment of a time multiplexed multiple threshold sensing system with a hysteresis logic circuit;

FIG. 8 is a schematic of an embodiment of a hysteresis logic circuit in accordance with the present invention;

FIG. 9 is a block diagram of an embodiment of a multiple threshold sensing system in accordance with the present invention;

FIG. 10 is a schematic of an embodiment of a multiple threshold temperature sensor circuit in accordance with the present invention;

FIG. 11 is a schematic of an embodiment of a multiple detect logic circuit in accordance with the present invention; and

FIG. 12 is a timing/temperature multiplexed diagram of multiple temperature threshold sensing of the multiple threshold sensing system of FIG. 9.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is best understood by comparison with the prior art. Hence, this detailed description begins with a discussion of known thermal sensing system 10 which includes hysteresis as shown in FIG. 1. Circuit 10 includes a current source implemented by a current mirror 12 which receives the input bias signal, I_(bias), from a current reference (not shown) having a bias current proportional to absolute temperature. Current mirror 12 includes transistors M₁ and M₂. The gates of transistors M₁ and M₂ couple to the drain of transistor M₁, which receives the input bias signal I_(bias). The sources of transistors M₁ and M₂ are grounded. Current mirror 12 is coupled to a multiple output current mirror 14. The multiple output current mirror 14 includes transistors M₃, M₄, M₅ and M₆. The gates of transistors M₃, M₄, M₅ and M₆ couple to the drain of transistor M₃. The current generated at the drains of transistors M₄, M₅ and M₆ provide three output currents. The source of a MOS transistor M₇ is coupled to the drain of transistor M₄. The drain of transistor M₇ is connected to the drain of transistor M₅ and the base of a bipolar transistor Q₁. A resistor R₁ is coupled between the base of bipolar transistor Q₁ and ground. The collector of transistor Q₁ is coupled to a drain of transistor M₆ to form node A. Transistor Q₁ serves as the thermal sense element of the thermal sensing system 10. The base-emitter voltage of transistor Q₁ has a negative temperature coefficient of approximately 1.5 to 2.5 mV/° C.

Node A is coupled to a first inverting output driver 16. The inverting output driver 16 includes transistors M₈ and M₉. The gates of both CMOS transistors M₈ and M₉ are coupled together at node A. The drains of both transistors M₈ and M₉ are coupled together to form node B. The source of transistor M₈ is coupled to the power supply rail V_(CC), while the source of transistor M₉ is grounded. An inverter 18 is coupled to the first inverting output driver 16 at node B. Inverter 18 includes transistors M₁₀ and M₁₁. The gates of both transistors M₁₀ and M₁₁, are coupled together at node B. The drains of both transistors M₁₀ and M₁₁, are coupled together. The source of transistor M₁₀ is coupled to the power supply rail V_(CC), while the source of transistor M₁₁ is grounded. The gate of transistor M₇ is connected to the node C, the node common to the drains of both transistors M₁₀ and M₁₁. The inverting output driver 16 generates a detection signal D₁ drawn from node B.

Initially, during a first mode of operation, transistors M₁, M₂, M₃, M₄, M₅, and M₆ are conducting current and transistors M₉ and M₁₀ are on, while transistors Q₁, and M₇ are not conducting current and transistors M₈ and M₁₁, are off. External to the thermal sensing circuit 10, a current reference is generated with a current reference (not shown) having a bias current proportional to absolute temperature. The a current reference having a bias current proportional to absolute temperature provides a bias current I_(bias). Current I₅ is applied to the base of transistor Q₁ and across resistor R₁. Prior to the threshold voltage of transistor Q₁, the voltage across the resistor R₁ equals the current multiplied by the resistance of R₁. After the voltage applied to the base of transistor Q₁ exceeds the threshold voltage, the voltage across resistor R₁ is represented by the following equation:

V=kT/qIn (I _(ref) /I _(sat))

where k represents Boltzmann's constant, T is the temperature, q is the charge of an electron, current I_(sat) is the saturation current and current I_(ref) is the reference current or the collector current I_(C1). According to this equation, as temperature increases, the base-emitter voltage V_(be) necessary to turn on the transistor Q₁ decreases.

The bias current I_(bias) is set such that when the temperature of the integrated circuit equals the upper threshold temperature t₁, transistor Q₁ begins to conduct current. A typical pre-determined temperature threshold t₁ is 150° C. When the temperature of the integrated circuit reaches the upper threshold temperature t₁, transistor Q₁ begins to conduct more current than transistor M₆ can supply and pulls node A low. As a result, transistor M₈ turns on and transistor M₉ turns off, pulling the detection signal D₁ high. At this point, detection signal D₁ may be used to shut down a particular function or perform some other function indicating that the integrated circuit temperature has reached the upper temperature threshold t₁.

As noted above, transistors M₁₀ and M₁₁ define the inverter 18 which is used to apply voltage to the gate of transistor M₇ sufficient to enable or disable transistor M₇ from conducting current when either the upper or lower predetermined temperature thresholds are reached. Accordingly, together transistors M₄, M₇, M₁₀ and M₁₁, form a hysteresis circuit. In a second mode of operation, while detection signal D₁ is high, transistor M₁₀ turns off and transistor M₁₁, turns on. Accordingly, node C goes low, enabling transistor M₇ to conduct current. Circuit 10 either shuts down a particular function of the integrated circuit or performs some other function indicating that the integrated circuit temperature has reached the upper temperature threshold t₁. At this point, if the integrated circuit is shut down, the integrated circuit begins to cool. Simultaneously, circuit 10 continues to monitor and detect when the temperature of the integrated circuit has reached a lower temperature threshold point t₀. As long as the temperature of the integrated circuit remains between the upper and the lower temperature threshold points, t₀ and t₁, the state of the thermal sense circuit continues to exist as it did at the end of the first mode as explained in the previous paragraph. Particularly, the detection signal D₁ remains high, transistors Q₁, M₁, M₂, M₃, M₄, M₅, M₆, and M₇, continue to conduct current and transistors M₈ and M₁₁, are on, while transistors M₉ and M₁₀ are off.

Since transistor M₇ conducts current, the current from transistor M₄ flows through transistor M₇ and currents 15 and 17 are applied to the base of transistor Q₁. This cumulative current alters the base-emitter voltage V_(BE) of transistor Q₁ to be the voltage sufficient to turn on transistor Q₁ when the temperature of the integrated circuit equals the lower threshold temperature t₀. Thus, the moment the temperature of the integrated circuit falls to the lower threshold temperature t₀, transistor Q₁ ceases to conduct current. Transistor M₆ pulls node A high. Transistor M₈ turns off and transistor M₉ turns on, pulling the detection signal D₁ low. At this point, detection signal D₁ may be used to turn the once shut down integrated circuit back on. Transistor M₁₀ turns on and transistor M₁₁ turns off. Node C goes high, turning transistor M₇ off. At this point, the thermal sensing circuit 10 returns to the first mode of operation, detecting a rise in temperature at the upper temperature threshold t₁ as explained above. The circuit 10 continues to operate in this cyclic fashion.

The disadvantage with this embodiment is that there is no way to determine exactly when the temperature of the integrated circuit equals the lower temperature threshold to when the temperature is rising. In addition, it is not always desirable to shut down a integrated circuit.

This thermal sense circuit that protects an integrated circuit from overheating is successful; however, in many systems, it is desirable to keep all functions available to the end-user. Setting a thermal sense flag eliminates the need to shut down the function entirely. This thermal sense flag can be used by external or internal circuitry to selectively reduce the power consumption of the integrated circuit in an effort to prevent overheating. As depicted in FIG. 2, a second thermal sense circuit 24 with a lower detect threshold creates this feature. The thermal sense system 20 includes two thermal sense circuits 22 and 24. These circuits 22 and 24 are equivalent in design to the thermal sensing circuit 10 as shown in FIG. 1. The input bias signal I_(bias1) is coupled to the input of the first thermal sensing circuit 22. The input bias signal I_(bias2) is coupled to the input of the second thermal sensing circuit 24. The bias current signals, I_(bias1), and I_(bias2), propagate from either the same current reference or two separate current references (not shown). The first thermal sensor circuit 22 generates a first detection signal D₂ while the second thermal sensing circuit 24 generates a second detection signal D₃.

The use of separate sense elements, however, introduces the problem of matching the process, die stress, and thermal gradients between the two sense elements. Additional die area for the redundant current mirroring required for multiple circuits increases cost and power consumption. The problem is excessive when sensing two or more distinct thresholds.

The thermal sense system 30 is an embodiment in accordance with the present invention as shown in FIG. 3 includes a thermal sensing circuit 40 and a decode circuit 50 having two detect latches. The detection clock signal C₁ runs at a higher frequency than the temperature selection clock signal T₁. For example, detection clock signal C₁ can be ten times as fast as the temperature selection clock signal T₁. The input signal I_(bias3) is coupled to the thermal sensing circuit 40 at its input for providing a voltage bias for the thermal sense element of thermal sensing circuit 40. The temperature selection clock signal T₁ is coupled to the thermal sensing circuit 40 and the decode circuit 50 for selectively clocking the two temperature detection modes of operation. The thermal sensing circuit 40 generates a detection signal D₄. This detection signal D₄ signal couples to the decode circuit 50 at is input. The decode circuit determines which threshold temperature has been reached, either the first temperature threshold t₂ or the second temperature threshold t₃. The detection clock signal C₁ is coupled to the decode circuit 50 to provide a clocking signal. The decode circuit 50 generates two output signals DT₁ and DT₂. These signals coupled to the hysteresis logic circuit 60 (to be discussed in a later paragraph with regard to FIG. 8) to generate a shutdown flag, S₁.

The thermal sensing circuit 40 in FIG. 4 includes a current source implemented by a first current mirror 42, multiple output current mirror 44, a MOSFET transistor M₄₈, a resistor R₂, a bipolar transistor Q₁₀, and an inverting output driver 46. Current mirror 42 receives the input bias signal, I_(bias3). Current mirror 42 includes transistors M₄₀ and M₄₂. The gates of transistors M₄₀ and M₄₂ couple to the drain of transistor M₄₀, which receives the input bias signal I_(bias3). The sources of transistors M₄₀ and M₄₂ are grounded. Current mirror 42 is coupled to the multiple output current mirror 44. The multiple output current mirror 44 includes transistors M₄₄, M₄₆, M₅₀and M₅₂. The gates of transistors M₄₄, M₄₆, M₅₀ and M₅₂ couple to the drain of transistor M₄₄. The drain of transistor M₄₄ couples to the drain of transistor M₄₂. The sources of transistors M₄₄, M₄₆, M₅₀ and M₅₂ couple to a power supply V_(CC). The current generated at the drains of transistors M₄₈, M₅₀ and M₅₂ provide three output currents. The source of transistor M₄₈ is coupled to the drain of transistor M₄₆, while the gate of transistor M₄₈ is coupled to the temperature selection signal T₁. The drain of transistor M₄₈ is connected to the drain of transistor M₅₀ and the base of a bipolar transistor Q₁₀. A resistor R₂ is coupled between the base of bipolar transistor Q₁₀ and ground. The collector of transistor Q₁₀ is coupled to a drain of transistor M₅₂ to form node D. Transistor Q₁₀ serves as the thermal sense element of the thermal sensing system 40. Transistor Q₁₀ has a negative temperature coefficient of approximately 1.5 to 2.5 mV/° C.

Node D is coupled to the inverting output driver 46. The inverting output driver 46 includes transistors M₅₄ and M₅₆. The gates of both transistors M₅₄ and M₅₆ are coupled together at node D. The drains of both transistors M₅₄ and M₅₆ are coupled together to form node E. The source of transistor ₅₄ is coupled to the power supply rail V_(CC), while the source of transistor M₅₆ is grounded. The inverting output driver generates a detection signal D₄ at node E.

Initially, during a first mode of operation, transistors M₄₀, M₄₂, M₄₄, M₄₆, and M₅₀, are conducting current and transistors M₅₂, and M₅₆ are on, while transistors Q₁₀, and M₄₈ are not conducting current and transistor M₅₄ is off. External to the thermal sensing circuit 40, a voltage reference is generated with a current reference (not shown) having a bias current proportional to absolute temperature. The a current reference having a bias current proportional to absolute temperature provides a bias current I_(bias3) Current I₅₀ is applied to the base of transistor Q₁₀ and across resistor R₂. Prior to the threshold voltage of transistor Q₁₀, the voltage across the resistor R₂ equals the current multiplied by the resistance of R₂. After the voltage applied to the base of transistor Q₁₀ exceeds the threshold voltage, the voltage across resistor R₂ is represented by the following equation:

V=kT/qIn(I _(ref/) I _(sat))

where k represents Boltzmann's constant, T is the temperature, q is the charge of an electron, current I_(sat) is the saturation current and current I_(ref) is the reference current or the collector current I_(C10). As temperature increases, the base-emitter voltage V_(be) necessary to turn on the transistor Q₁₀ decreases. A typical pre-determined temperature threshold t₁ is 150° C.

By setting a proper ratio of current mirrors and resistance in the thermal sensing circuit 40, transistor Q₁₀ begins to conduct current at the desired temperature. The definition of the size of resistor R₂ and transistors M₅₀ and M₄₆ create the two temperature threshold points: an upper temperature threshold t₃ and a lower temperature threshold t₂. An example may include the use of two threshold points, an upper at 150° C. and a lower at 125° C.

The bias current I_(bias3) is set such that when the temperature of the integrated circuit equals the upper threshold temperature t₃, transistor Q₁₀ begins to conduct current. When the temperature of the integrated circuit reaches the upper threshold temperature t₃, transistor Q₁₀ begins to conduct current and pulls node D low. As a result, transistor M₅₄ turns on and transistor M₅₆ turns off, pulling the detection signal D₄ high. At this point, detection signal D₄ may be used to shut down a particular function or perform some other function indicating that the integrated circuit temperature has reached the upper temperature threshold t₃.

The temperature selection signal T₁ controls when the circuit detects the upper or lower temperature thresholds, t₃ and t₂, respectively. The temperature selection clock signal T₁ toggles between zero and one to adjust the current applied to the base of transistor Q₁₀. When the current applied to the base of transistor Q₁₀ increases, the base-emitter voltage V_(be) necessary to turn on the transistor Q₁₀ decreases. Accordingly, the lower temperature threshold t₂ is set thereby. Specifically, when the temperature selection clock signal T₁ toggles to zero, the lower temperature threshold t₂ is set. When the temperature selection clock signal T₁ toggles to one, the upper temperature threshold t₃ is set. As a result, the temperature selection clock signal T₁ determines at which temperature the detection signal D₄ will go high. For the circuit shown, when temperature selection clock signal T₁ is high, transistor M₄₈ is not conducting current. The current through transistor M₅₀ is applied across resistor R₂. The thermal voltage for transistor Q₁₀ corresponds to the upper temperature threshold t₃. The detection signal D₄ will go high when temperature equals this upper temperature threshold t₃. When the temperature selection clock signal T₁ is low, transistor M₄₈ is turned on and the current through both transistors M₅₀ and M₄₈ are applied across resistor R₂, raising the voltage across resistor R₂. Thus, the thermal voltage for transistor Q₁₀ decreases to the lower temperature threshold t₂. The detection signal D₄will go high when temperature equals this lower temperature threshold t₂.

A schematic for the decode circuit 50 with two detect latches is shown in FIG. 5. The decode circuit 50 includes three inverters 52, 58, and 60, two NAND gates 54 and 56 and two flip-flop registers 62 and 64. Inputs to the detect circuit 50 include the detection signal D₄ from the thermal sensing circuit 40, temperature selection clock signal T₁, and a detection clock signal C₁. The first NAND gate 54 receives two input signals: the detection clock signal C₁ and the temperature selection clock signal T₁. The output of the first NAND gate 54 couples to the input of inverter 58. The output of inverter 58 couples to the clock input of the flip-flop register 62. The second NAND gate 56 receives the detection clock signal C₁ and an inverted temperature selection clock signal T₁, which is inverted by inverter 52. The output of the second NAND gate 56 is coupled to the inverter 60. The output of the inverter 60 couples to the clock signal input of flip-flop register 64. The detection signal D₄ is coupled to the input ports of both flip-flop registers 62 and 64. Flip-flop registers 62 and 64 produce output signals DT₁ and DT₂, respectively. These output signals, DT₁ and DT₂, correspond to lower and upper temperature thresholds, t₂ and t₃, respectively. The temperature selection clock signal T₁ is used to mask or pass the detection clock signal C₁ to the detection latches, DT₁ and DT₂. If the detection signal D₄ is high while the temperature selection clock signal T₁ is low, then the detection latch DT₁ is set. If detection signal D₄ is high while temperature selection clock signal T₁ is high, then the detection latch DT₂ is set.

By alternating temperature selection clock signal T₁ at some frequency (i.e. 25 kHz) and detection clock signal C₁ at a sufficiently higher frequency (i.e. 200 kHz), the thermal latch outputs DT₁ and DT₂ can be decoded to determine whether the die is below the lower temperature threshold t₂, between upper and lower temperature thresholds, t₃ and t₂, or greater than the upper temperature threshold t₃. With thermal latch DT₁ as the most significant bit (MSB) and thermal latch DT₂ as the least significant bit (LSB), these temperature ranges correspond to ‘00’, ‘01’, ‘10’, and ‘11’ appearing on the detection latch outputs, DT₁ and DT₂.

A diagram demonstrating the time multiplexed dual temperature threshold sensing with a single element according to FIG. 3 is shown in FIG. 6. As can be seen, when the temperature selection clock signal T₁ toggles to zero, the lower temperature threshold is set. When the temperature selection clock signal T₁ toggles to one, the upper temperature threshold is set. As a result, the temperature selection clock signal T₁ determines at which temperature the detection signal D₄ will go high. Consequently, when the temperature selection clock signal T₁ is low, the detection signal D₄ will go high when temperature equals this lower temperature threshold t₂. When temperature selection clock signal T₁ is high, the detection signal D₄ will go high when temperature equals this upper temperature threshold t₃. The temperature selection clock signal T₁ is used to mask or pass the detection clock signal C₁ to the detection latches, DT₁ and DT₂. If the detection signal D₄ is high while the temperature selection clock signal T₁ is low, then the detection latch DT₁ is set. If detection signal D₄ is high while temperature selection clock signal T₁ is high, then the detection latch DT₂ is set.

FIG. 7 depicts an embodiment according to the present invention that includes a hysteresis logic circuit 60 coupled to decode circuit 50. The temperature selection clock signal T₁ and the detection latch outputs, DT₁ and DT₂ couple to the hysteresis logic circuit 60 to generate a shutdown flag, S₁.

The hysteresis logic circuit 60 as shown in FIG. 8 includes an inverter 66, two NOR gates 68 and 70, and a flip-flop register 72. The inverter 66 receives the thermal latch signal DT₂. The NOR gate 68 receives the thermal latch signal DT₁ and the shutdown signal, S₁. The outputs of inverter 66 and NOR gate 68 are coupled to the inputs of NOR gate 70. The output of NOR gate 70 is fed into the input of flip-flop register 72 and the temperature selection signal T₁ into the clock signal input. The flip-flop register 72 generates a thermal shutdown signal, S₁. This signal is fed back to one input of NOR gate 68 as noted above. Thermal shutdown occurs when both latch signals DT₁ and DT₂ have been detected. However, to implement hysteresis, the shutdown bit cannot be cleared until both the detection latches signals, DT₁ and DT₂, are cleared. Referring back to FIG. 6, note that when both detection latches signals, DT₁ and DT₂, are high, the thermal shutdown signal S₁ is set.

The two temperature threshold circuits can easily be modified for multiple thresholds by adding a threshold switch, a current source, and a detect latch for each temperature threshold as shown in FIG. 9. Specifically, the thermal sensor system 80 includes a thermal sense circuit 90, a decoder 92, and a detect circuit 100. The thermal sense circuit 90 receives the input bias signal, I_(bias4), and two temperature selection clock signals, TS₀ and TS₁, to generate a detection signal D₅. Decoder 92 decodes the two temperature selection clock signals TS₀ and TS₁ into signals T₂, T₃, T₄, and T₅. The detection clock signal C₂ provides the clocking for signals of the circuit 80. Detection signal D₅ and signals T₂, T₃, T₄, and T₅ are fed into the detection circuit 100 to generate threshold latch signals DT₃, DT₄, DT₅, and DT₆. By synchronous decoding of the thermal sensing circuit 90 and setting of the threshold latch signals DT₃, DT₄, DT₅, and DT₆, the die temperature or relative die temperature change can be determined within the ranges of the various sense thresholds.

The thermal sense circuit 90 is shown in FIG. 10. Circuit 90 includes a current source implemented by a current mirror 92, multiple output current mirror 94, a first MOS transistor M₉₈, a second MOS transistor M₁₀₂, a bipolar transistor Q₂₀, a resistor R₃ and an inverting output driver 96. Current mirror 92 includes transistors M₉₀ and M₉₂. The gates of transistors M₉₀ and M₉₂ couple to the drain of transistor M₉₀, which receives the input bias signal I_(bias4). The sources of transistors M₉₀ and M₉₂ are grounded. Current mirror 92 is coupled to the multiple output current mirror 94. Multiple output current mirror 94 includes transistors M₉₄, M₉₆, M₁₀₀ M₁₀₄ and M₁₀₆. The gates of transistors M₉₄, M₉₆, M₁₀₀ M₁₀₄ and M₁₀₆ couple to the drain of transistor M₉₄. The drain of transistor M₉₄ couples to the drain of transistor M₉₂. The sources of transistors M₉₄, M₉₆, M₁₀₀ M₁₀₄ and M₁₀₆ couple to a power supply V_(CC). The current generated at the drains of transistors M₉₆, M₁₀₀ M₁₀₄ and M₁₀₆ provide four output currents. The source of transistor M₉₈ is coupled to the drain of transistor M₉₆, while the gate of transistor M₉₈ is coupled to the temperature selection signal TS₁. The source of transistor M₁₀₂ is coupled to the drain of transistor M₁₀₀, while the gate of transistor M₁₀₂ is coupled to the temperature selection signal TS₀. The drains of transistors, M₉₈, M₁₀₂, and M₁₀₄, are connected to the base of a bipolar transistor Q₂₀. A resistor R₃ is coupled between the base of bipolar transistor Q₂₀ and ground. The collector of transistor Q₂₀ is coupled to a drain of transistor M₁₀₆ to form node F. Transistor Q₂₀ serves as the thermal sense element of the thermal sensing system 90. Transistor Q₂₀ has a negative temperature coefficient of approximately 1.5 to 2.5 mV/° C.

Node F is coupled to the inverting output driver 96. The inverting output driver 96 includes transistors M₁₀₈ and M₁₁₀. The gates of both transistors M₁₀₈ and M₁₁₀ are coupled together at node F. The drains of both transistors M₁₀₈ and M₁₁₀ are coupled together to form node G. The source of transistor M₁₀₈ is coupled to the power supply rail V_(CC), while the source of transistor M₁₁₀ is grounded. The inverting output driver generates a detection signal D₅ at node G.

Initially, during a first mode of operation, transistors M₉₀, M₉₂, M₉₄, M₉₆, M₁₀₀, M₁₀₄, and M₁₀₆ are conducting current and transistor M₁₁₀ is on, while transistors Q₂₀ M₉₈, and M₁₀₂ are not conducting current and transistor M,₁₀₈ is off. External to the thermal sensing circuit 90, a voltage reference is generated with a current reference (not shown) having a bias current proportional to absolute temperature. The a current reference having a bias current proportional to absolute temperature provides a bias current I_(bias4) Current I_(b) is applied to the base of transistor Q₂₀ and across resistor R₃. Prior to the threshold voltage of transistor Q₂₀, the voltage across the resistor R₃ equals the current multiplied by the resistance of R₃. After the voltage applied to the base of transistor Q₂₀ exceeds the threshold voltage, the voltage across resistor R₃ is represented by the following equation:

 V=kT/qIn(I _(ref/) I _(sat))

where k represents Boltzmann's constant, T is the temperature, q is the charge of an electron, current I_(sat) is the saturation current and current I_(ref) is the reference current or the collector current I_(C20). As temperature increases, the base-emitter voltage V_(be) necessary to turn on the transistor Q₂₀ decreases. By setting a proper ratio of current mirrors and resistance in the thermal sensing circuit 90, transistor Q₂₀ begins to conduct current at the desired temperature. The definition of the size of resistor R₃ and transistors M₉₈, M₁₀₂, and M₁₀₄ create the two temperature threshold points of t₂, t₃, t₄ and t₅.

The bias current I_(bias4) is set such that when the temperature of the integrated circuit equals the upper threshold temperature t₅, transistor Q₂₀ begins to conduct current. Thus, when the temperature of the integrated circuit reaches the upper threshold temperature t₅, transistor Q₂₀ begins to conduct current and pulls node F low. As a result, transistor M,₁₀₈ turns on and transistor M₁₁₀ turns off, pulling the detection signal D₅ high.

The temperature selection signals TS₁ and TS₂ control when the circuit detects the temperature thresholds, t₂, t₃, t₄ and t₅. Both temperature selection clock signals TS₁ and TS₂ toggle between zero and one to adjust the current applied to the base of transistor Q₂₀. When the current applied to the base of transistor Q₂₀ increases, the base-emitter voltage V_(be) necessary to turn on the transistor Q₂₀ decreases. Specifically, there are four states given the combination of the two temperature selection clock signals TS₀ and TS₁: ‘00’, ‘01’, ‘10’, ‘11’; where the most significant bit is TS₁ and the least significant bit is TS₀.

Given that signals TS₀ and TS₁ are both zero, the lower temperature threshold t₂ is set. Transistors M₉₈ and M₁₀₂ are conducting current. Therefore, current through transistors M₉₈ and M₁₀₂ combine to form current I_(a) Accordingly, current I_(b) through transistor M₁₀₄ combines with I_(a) to be applied to the base of transistor Q₂₀. At this point, the thermal voltage for transistor Q₂₀ corresponds to the lower temperature threshold t₂. Thus, the detection signal D₅ will go high when the temperature of the integrated circuits equals the lower temperature threshold t₂.

Given signal T_(D) is a one and signal TS₁ is a zero, the temperature threshold t₃ is set. Transistors M₉₈ is conducting current and M₁₀₂ is not conducting current. Current I_(a) comprises the current through transistor M₉₈ solely. When the combined currents of I_(b) and I_(a) are applied to the base of transistor Q₂₀, the thermal voltage of transistor Q₂₀ corresponds to the temperature threshold t₃. Detection signal D₅ will go high when the temperature of the integrated circuits equals the lower temperature threshold t₃.

When signal TS₀ is a zero and signal TS₁ is a one, the temperature threshold t₄ is set. Transistors M₉₈ is not conducting current and M₁₀₂ is conducting current. Current I_(a) comprises the current through transistor M₁₀₂. When the combined currents of I_(b) and I_(a) are applied to the base of transistor Q₂₀, the thermal voltage of transistor Q₂₀ corresponds to the temperature threshold t₄. Detection signal D₅ will go high when the temperature of the integrated circuits equals the lower temperature threshold t₄.

Given signals that TS₀ and TS₁ are both one, the upper temperature threshold t₅ is set. Transistors M₉₈ and M₁₀₂ are not conducting current and I_(a) is 0. When current I_(b) is applied to the base of transistor Q₂₀, the thermal voltage of transistor Q₂₀ corresponds to the temperature threshold t₅. Detection signal D₅ will go high when the temperature of the integrated circuits equals the lower temperature threshold t₅.

The decode circuit 100 as shown in FIG. 11 includes four NAND gates 102, 104, 106, and 108, four inverters 110, 112, 114 and 116, and four flip flop registers 118, 120, 122, and 124. Signals TS₀ and TS₁ are applied to the decoder 92 (shown in FIG. 9) to generate decoded temperature selection signals T₂, T₃, T₄, and T₅. Signals T₂, T₃, T₄, and T₅ correspond to temperature thresholds t₂, t₃, t₄, and t₅. The detection clocking signal C₂ and the temperature selection signal T₂ are coupled to the inputs of NAND gate 102. The detection clocking signal C₂ and the temperature selection signal T₃ are coupled to the inputs of NAND gate 104. The detection clocking signal C₂ and the temperature selection clock signal T₄ are coupled to the inputs of NAND gate 106. The detection clocking signal C₂ and the temperature selection clock signal T₅ are coupled to the inputs of NAND gate 108. The outputs of NAND gates 102, 104, 106, and 108 are fed into inverters 110, 112, 114 and 116 respectively. The outputs of inverters 110, 112, 114 and 116 are fed into the clock inputs of flip flop registers 118, 120, 122 and 124, respectively. The detection signal D₅ is fed into the input of each flip flop register, 118, 120, 122, and 124. Flip flop registers 118, 120, 122, and 124 generate output signals DT₃, DT₄, DT₅, and DT₆, respectively.

A timing diagram demonstrating the time multiplexed dual temperature threshold sensing with a single element according to FIG. 9 is shown in FIG. 12. The temperature selection signals TS₁ and TS₂ control when the circuit detects the temperature thresholds, t₂, t₃, t₄ and t₅. Specifically, there are four states given the combination of the two temperature selection clock signals TS₀ and TS₁: ‘00’, ‘01’, ‘10’, ‘11’. Given signals TS₀ and TS₁ are both zero, the lower temperature threshold t₂ is set. Detection signal D₅ will go high when the temperature of the integrated circuits equals the lower temperature threshold t₂. Detection latch DT₃ will go high as well. When signal TS₀ is a one and signal TS₁ is a zero, the temperature threshold t₃ is set. Detection signal D₅ will go high when the temperature of the integrated circuits equals the temperature threshold t₃. Accordingly, detection latch DT₄ will go high. When signal TS₀ is a zero and signal TS₁ is a one, the temperature threshold t₄ is set. Detection signal D₅ will go high when the temperature of the integrated circuits equals the temperature threshold t₄. In addition, detection latch DT₅ will go high. Given signals TS₀ and TS₁ are both one, the upper temperature threshold t₅ is set. Detection signal D₅ will go high when the temperature of the integrated circuits equals the temperature threshold t₅. As a result, detection latch DT₆ will go high.

This flexible on-board temperature monitoring solution reduces the cost of thermal feedback sensing by reducing die area and improves the correlation of detected temperatures. In addition, this solution reduces the possibility of mismatch and tracking errors between two or more sense elements.

Further scope of applicability of the present invention should become apparent from the detailed description given above. However, it should be understood that the detailed description and specific examples, while indicating embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention should become apparent to those skilled in the art from this detailed description. Accordingly, this detailed description and specific examples are not to be considered as limiting the present invention. 

What is claimed is:
 1. A thermal sensing system for an integrated circuit comprising: a thermal sensing circuit, having a first input for a voltage bias and a second input for a temperature selection clock signal T₁, the thermal sensing circuit for monitoring the temperature t₀ of the integrated circuit for a temperature in between a first t₁ and a second t₂ threshold temperature range, wherein if t₁≦t₀≦t₂, the thermal sensing circuit generates at least one detection signal; a decode circuit, having a first input, a second input, a first detect latch output, and a second detect latch output, the decode circuit coupled to the thermal sensing circuit for receiving the at least one detection signal to determine whether the first t₁ and second t₂ threshold temperature has been reached and generating at least one detection flag signal at a predetermined threshold temperature, the first input coupled to receive the temperature selection clock signal T₁, the second input coupled to receive the a detection clock signal C₁, wherein the detection clock signal C₁ has a higher frequency than that of the temperature selection clock signal T₁; and a hysteresis circuit coupled to the first and second detect latch outputs for receiving the at least one detection flag and generating a shutdown signal to selectively reduce the power consumption of the integrated circuit to prevent overheating.
 2. The thermal sense system according to claim 1, wherein the thermal sense circuit includes a current source; a multiple output current mirror having an input lead and at least three output leads, the input lead coupled to the current source; a MOSFET transistor having a gate, a source and a drain, the source coupled to the first output of the multiple output current mirror for receiving current; a bipolar transistor having a base, collector, and emitter, the base coupled to the second output of the multiple output current mirror and drain of the MOSFET transistor, the collector coupled to the third output of the multiple output current mirror; a resistor coupled between the base of the bipolar transistor and ground; and an inverting output driver for generating the detection signal, the inverting output driver coupled to the collector of the bipolar transistor.
 3. The thermal sense circuit according to claim 2, wherein the multiple output current mirror having an input and three outputs includes a first MOS transistor having a source, drain and gate, the source coupled to the power supply rail, the drain and gate coupled to the input of the multiple output current mirror; a second MOS transistor having a source, drain and gate, the source coupled to the power supply rail, the drain coupled to the first output of the multiple output current mirror, the gate coupled to the gate of the first MOS transistor; a third MOS transistor having a source, drain and gate, the source coupled to the power supply rail, the drain coupled to the second output of the multiple output current mirror, the gate coupled to the gate of the first MOS transistor; and a fourth MOS transistor having a source, drain and gate, the source coupled to the power supply rail, the drain coupled to the third output of the multiple output current mirror, the gate coupled to the gate of the first MOS transistor.
 4. The thermal sense circuit according to claim 2, wherein the inverting output driver includes a first MOS transistor having a source, drain and gate, the source coupled to the power supply rail and a second MOS transistor having a source, drain and gate, the source coupled to ground, the gate coupled to the gate of the first MOS transistor, the drain coupled to the drain of the first MOS transistor to form a detection signal node.
 5. The thermal sensing system according to claim 1, wherein the decode circuit includes a first and second NAND gate, each NAND gate having at least two inputs and at least one output, the first input of the first NAND gate coupled to the detection clock signal, the second input of the first NAND gate coupled to the temperature selection clock signal, the first input of the second NAND gate coupled to the first input of the first NAND gate; a first inverter having an input and an output, the input coupled to the temperature selection signal, the output coupled as the second input of the second NAND gate; a second and third inverter each having an input and an output, the input of the second inverter coupled to the output of the first NAND gate, the input of the third inverter coupled to the output of the second NAND gate; and a first and a second flip-flop register each having a clocking input, an signal input and an output for generating a detection flag signal, the clocking input of the first flip-flop register coupled to the output of the second inverter, the signal input of the first flip-flop register coupled to the detection signal, the clocking input of the second flip-flop register coupled to the output of the third inverter, the signal input of the second flip-flop register coupled to the detection signal.
 6. The thermal sensing system according to claim 1, wherein the hysteresis circuit includes: a first inverter having an input and an output, the input coupled to the output of the first flip-flop register to receive the detection flag signal of the first flip-flop register; a first NOR gate having at least two inputs and one output, the first input coupled to the output of the second flip-flop register to receive the detection flag signal of the first flip-flop register; a second NOR gate having at least two inputs and one output, the first input coupled to the output of the first inverter, the second input coupled to the output of the first NOR gate; and a third flip-flop register having a clocking input, an signal input and an output for generating a shutdown signal, the clocking input coupled to receive the temperature selection signal, the signal input coupled to receive the output of second NOR gate, the output coupled to the second input of the first NOR gate.
 7. A thermal sensing system for an integrated circuit comprising: a thermal sensing circuit coupled to receive at least two temperature selection signals of predetermined threshold temperatures and bias current for monitoring the temperature of the integrated circuit and generating at least one detection signal; a decoder for decoding the temperature selection signals coupled to receive the temperature selection signals and generating decoded temperature selection signals; and a decode circuit coupled to said thermal sensing circuit for receiving the at least one detection signal, the decoded temperature selection signals, and a detection clocking signal, the decode circuit for generating corresponding detection flag signals at the predetermined threshold temperatures.
 8. The thermal sense system according to claim 7, wherein the thermal sense circuit includes: a current source; a multiple output current mirror having an input lead and at least four output leads, the input lead coupled to the first current mirror; a first and second MOS transistor having a gate, a source and a drain, the source of the first MOS transistor coupled to the first output of the multiple output current mirror for receiving current, the source of the second MOS transistor coupled to the second output of the multiple output current mirror for receiving current, gate of the first MOS transistor coupled to receive the first temperature selection signal, gate of the second MOS transistor coupled to receive the second temperature selection signal; a bipolar transistor having a base, collector, and emitter, the base coupled to the third output of the multiple output current mirror and drain of the first and second MOS transistors, the collector coupled to the fourth output of the multiple output current mirror; a resistor coupled between the base of the bipolar transistor and ground; and an inverting output driver for generating the detection signal, the inverting output driver coupled to the collector of the bipolar transistor.
 9. The thermal sense circuit according to claim 8, wherein the multiple output current mirror having an input and at least four outputs includes a first MOS transistor having a source, drain and gate, the source coupled to the power supply rail, the drain and gate coupled to the input of the multiple output current mirror; a second MOS transistor having a source, drain and gate, the source coupled to the power supply rail, the drain coupled to the first output of the multiple output current mirror, the gate coupled to the gate of the first MOS transistor; a third MOS transistor having a source, drain and gate, the source coupled to the power supply rail, the drain coupled to the second output of the multiple output current mirror, the gate coupled to the gate of the first MOS transistor; a fourth MOS transistor having a source, drain and gate, the source coupled to the power supply rail, the drain coupled to the third output of the multiple output current mirror, the gate coupled to the gate of the first MOS transistor; and a fifth MOS transistor having a source, drain and gate, the source coupled to the power supply rail, the drain coupled to the fourth output of the multiple output current mirror, the gate coupled to the gate of the first MOS transistor.
 10. The thermal sense circuit according to claim 8, wherein the inverting output driver includes a first MOS transistor having a source, drain and gate, the source coupled to the power supply rail and a second MOS transistor having a source, drain and gate, the source coupled to ground, the gate coupled to the gate of the first MOS transistor, the drain coupled to the drain of the first MOS transistor to form a detection signal node.
 11. The thermal sensing system according to claim 8, wherein the decode circuit includes: a first, second, third and fourth NAND gate, each NAND gate having at least two inputs and at least one output, each NAND gate having the first input coupled to the detection clock signal, the second input of the first NAND gate coupled to the first decoded temperature selection signal, the second input of the second NAND gate coupled to the second decoded temperature selection signal, the second input of the third NAND gate coupled to the third decoded temperature selection signal, the second input of the fourth NAND gate coupled to the fourth decoded temperature selection signal, the first input of the second NAND gate coupled to the first input of the first NAND gate; a first, second, third and fourth inverter having an input and an output, the input of the first inverter coupled to the output of the first NAND gate, the input of the second inverter coupled to the output of the second NAND gate, the input of the third inverter coupled to the output of the third NAND gate, the input of the fourth inverter coupled to the output of the fourth NAND gate; and a first, second, third and fourth flip-flop register each having a clocking input, an signal input and an output for generating a detection flag signal, each flip-flop register having the signal input coupled to the detection signal node, the clocking input of the first flip-flop register coupled to the output of the first inverter, the clocking input of the second flip-flop register coupled to the output of the second inverter, the clocking input of the third flip-flop register coupled to the output of the third inverter, the clocking input of the fourth flip-flop register coupled to the output of the fourth inverter. 